The 21-year-old missed Monday's 3-1 win over Brighton due to a small setback in his recovery from the hamstring injury he sustained in the Blues' FA Cup Final defeat to Arsenal on Aug. 1.
Pulisic had originally hoped to be fit to the start of the season but Lampard now claims the winger is facing a race against time to face the Premier League champions at Stamford Bridge on Sunday.
"Christian was training with us last week but he had a little bit of discomfort a couple of days ago where he was trying to train with us and be in contention for today," said Lampard after goals from Jorginho, Reece James and Kurt Zouma gave Chelsea victory at the Amex Stadium.
"It would have been a big ask for today but he's had to take a few steps back. That's one that I don't have big news on at the minute. We'll see."
Jorginho was limping and substituted late on, while Timo Werner had his right knee strapped with ice after the final. Lampard expects both to be available against Jurgen Klopp's side.
Lampard also had high praise for his German debutants Werner and Kai Havertz, and James in particular for his stellar go-ahead goal.
"It had me off my seat and we needed it, we weren't at our best in that period," Lampard said. "Reece has got that in him, he is a player of such quality. You try and get him in areas where he can be a threat.
"If you analyse the fact that we've been together as a team for four days, to expect everything to click on day one is difficult. We showed determination and that is a game we might have dropped points in last year."
Havertz, who worked hard in a three-pronged line behind Werner which included Ruben Loftus-Cheek and Mason Mount, was substituted in the 80th minute but Lampard was full of praise for the former Bayer Leverkusen forward.
"I liked Kai Havertz," said Lampard. "It's not a game where you come away with some vintage moments and there's a lot on his shoulders but you saw glimpses of what he can do.
"Everything I have seen from him is spot on and his quality will come through. He's hugely talented."
Lampard pointed out that despite the wealth of their attacking talent, Chelsea were still a work in progress and unlikely to challenge champions Liverpool for the title.
The Chelsea boss said: "We definitely want to close that gap to Liverpool, we have to have intentions to be up there even though it is a big ask to win it. We are behind, it's step-by-step. Hopefully we can make big strides."
Lampard also backed goalkeeper Kepa Arrizabalaga, who missed a save on the Brighton goal that he might have done better with, saying that the Spaniard was Chelsea's keeper and that he was very happy with him.